Toshiba plans to double the capacity of its Philippine HDD factory by fiscal year 2027 to capture AI data center storage demand. Following this announcement, shares of Seagate (STX) and Western Digital (WDC) both plummeted over 10% on Friday. The market previously widely believed that "supply discipline" in the hard disk drive (HDD) industry, meaning major manufacturers would not increase drive production capacity but only enhance capacity through technological improvements, was the core logic supporting stock price increases. Toshiba's expansion breaks this expectation, raising market concerns about medium-term supply risks.